The present invention relates generally to low pressure discharge fluorescent lamps providing white color illumination efficiently and more particularly to a novel phosphor combination producing such efficient illumination with improved color rendition. In providing said improvements, the present phosphor coating employs a calcium haloapatite phosphor base layer deposited on the inner wall of the lamp envelope over which is deposited a surface layer utilizing a particular two component phosphor blend.
The use of several phosphor materials in combination to produce a predetermined overall spectral energy distribution is well known. A recently introduced type of said lamp employs three phosphors each emitting in a relatively narrow spectral region to produce efficient white color illumination with a high degree of faithful color rendition. For example, in the U.S. Pat. No. 3,937,998 there is disclosed such a phosphor combination wherein a blended mixture of individual phosphors emitting in the blue, green and red color spectral regions combine to produce the desired white color lamp emission. The blue emission phosphor is said to be divalent europium activated barium and/or strontium magnesium aluminate having a hexagonal crystalline structure which can further include partial substitution of the magnesium ion with divalent manganese ion. The green emission phosphor is said to be a terbium activated aluminate having a hexagonal magnetoplumbite crystalline structure in which up to a maximum of 25 weight percent of aluminum may be replaced by gallium and/or scandium and magnesium may be entirely or partly replaced by zinc and/or beryllium as well as further include optional coactivation of said phosphor with cerium ion. The red emission phosphor in said phosphor combination is said to include rear earth oxides activated with trivalent europium wherein said rare earth ion is selected from at least one of the elements yttrium, gadolinium and lutetium. The composite lamp emission provided with said phosphor combination is said to provide color temperatures in the range from 2300.degree. K. to 8000.degree. K. with color rendering (CRI values) indices for said lamps being reported up to the value of 87. A multilayer coated fluorescent lamp using these phosphors is also disclosed in Japanese Pat. No. J54124-- 581. Said lamp employs a base layer of calcium halophosphate phosphor having a top coating deposited thereon formed with a phosphor mixture containing by weight 8-24% of europium-activated barium magnesium aluminate, 32-42% terbium-activated cerium magnesium aluminate, and 44-50% europium activated yttrium oxide. The coating weight ratio of the base layer to said top layer resides in the weight ration range 1:0.7-2.
In U.S. Pat. No. 4,005,328, there is still further disclosed a low pressure discharge fluorescent lamp employed for color copying by electrophotographic processes and which employs a single layer phosphor coating that is a blend of two of the above identified type phosphors. More particularly, said phosphor combination utilized various weight proportions of the europium-activated yttrium oxide phosphor or some other red color emitting rare earth phosphor and which is blended with a barium or strontium ternary aluminate coactivated with both manganese and europium ion to emit at limited wavelengths in the blue color and green color spectral regions. To match the sensitivity of the electrophotographic copying equipment, the blend proportions in said phosphor combination are said to be selected such that the proportion of the aluminate phosphor generally dominates.